Authors: Duran, A; Cardona Lopez, D

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Duran, A & Cardona Lopez, D 2020, 'Toe rock mass strength in footwall failures', in PM Dight (ed.), Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 749-762,

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This paper presents results of check and back-analysis of highwall slopes and footwall instabilities in coal bearing sedimentary strata. The primary aim being to address the issue of rock mass strength at the footwall toe. An issue highlighted previously by one of the authors is the often incomplete knowledge in a footwall scenario to enable analytically based design. Experience gained at coal mining operations in moderate to steeply dipping strata coupled with studies to resolve the unknowns has resulted in an understanding of the critical mechanism, shear strength on bedding defects, groundwater conditions and an approach to assessing the rock mass strength at the toe. This paper focuses on the latter aspect of toe rock mass strength. The results question the applicability of studies suggesting the use of the disturbance factor associated with unloading of slopes to sedimentary strata. The methodology proposed by the authors for the footwall toe rock mass strength to account for anisotropy is to accept a disturbance factor of zero and downrate the Geological Strength Index (GSI) value. Furthermore, the authors caution against the use of the GSI fissile molasse chart for coal bearing strata unless there is significant tectonic deformation of the rock mass evident at the small-scale.

Keywords: footwall, steep dips, coal mining, rock mass strengths, disturbance factor, back-analysis

Alejano, LR, Ferrero, AM, Ramırez-Oyanguren, P & Alvarez Fernandez, MI 2011, ‘Comparison of limit-equilibrium, numerical and physical models of wall slope stability’, International Journal Rock Mechanics and Mining Sciences, vol. 48, pp. 16–26.
Bieniawski, ZT 1989, Engineering Rock Mass Classifications A Complete Manual for Engineers and Geologists in Mining, Civil and Petroleum Engineering, John Wiley and Sons.
Cavers, DD, Baldwin, GJ, Hannah, T & Singhal, RK 1986, ‘Design methods for open pit coal mine footwalls’, Proceedings of the International Symposium on Geotechnical Stability in Surface Mining, pp. 79–86.
Cavers, DS 1981, ‘Simple methods to analyse buckling of rock slopes’, Rock Mechanics, vol. 14, pp. 87–104.
Coulthard, MA, Lucas, DS & Fuller, PG 2004, ‘Application of UDEC to a stress-related mine slope failure at Leigh Creek, South Australia’, International Symposium of Numerical Modelling of Discrete Materials in Geotechnical Engineering, Civil Engineering, and Earth Sciences, A.A. Balkema, Rotterdam, pp. 289–296.
Duran, A & Seedsman, RW 2018, ‘Managing water pressures in a steeply dipping coal–mine footwall’, Slope Stability 2018 – XIV International Congress on Energy and Mineral Resources, Asociación Nacional de Ingenieros de Minas, pp. 1436–1442.
Garzon, SER 2016, ‘Analytical solution for assessing continuum buckling in sedimentary rock slopes based on the tangent-modulus theory’, International Journal of Rock Mechanics and Mining Sciences, vol. 90, pp. 53–61.
Hawley, PM, Martin, DC & Acott, CP 1986, ‘Failure mechanics and design considerations for footwall slopes’, CIM Bulletin, vol. 79, iss. 896, pp. 47–53.
Hoek, E & Brown, ET 1980, ‘Empirical strength criterion’, Journal of the Geotechnical Engineering Division, vol. 106 (GT9),
pp. 1013–1035.
Hoek, E & Brown, ET 1997, ‘Practical estimates of rock mass strength’, International Journal of Rock Mechanics and Mining Sciences, vol. 34, iss. 8, pp. 1165–1186.
Hoek, E & Brown, ET 2018, ‘The Hoek-Brown failure criterion and GSI – 2018 edition’, Journal of Rock Mechanics and Geotechnical Engineering, vol. 11, iss. 3,
Hoek, E, Carranza-Torres, CT & Corkum, B 2002, ‘Hoek-Brown Failure Criterion— 2002 Edition’, Proceedings of the 5th North American Rock Mechanics Symposium, pp. 267–273.
Hoek, E, Marinos, PG & Marinos, VP 2005, ‘Characterisation and engineering properties of tectonically undisturbed but lithologically varied sedimentary rock masses’, International Journal of Rock Mechanics and Mining Sciences, vol. 42, iss. 2, pp. 277–285.
Krahn, J 2003, ‘The 2001 R.M. Hardy Lecture: The limits of limit equilibrium’, Canadian Geotechnical Journal, vol. 40, pp. 643–660.
Kutter, HK 1974, ‘Mechanisms of slope failure other than pure sliding’, in L Mueller (ed.), Rock Mechanics, CISM Courses and Lectures, p. 165, Springer, New York.
Mercer, KG 2012, ‘The history and development of the anisotropic linear model: part 1’, Newsletter Australian Centre for Geomechanics, Australian Centre for Geomechanics, vol. 38, July 2012, pp. 13‒16,
Rose, ND, Scholz, J, Burden, J, King, M, Maggs, C & Havaej, M 2018, ‘Quantifying transitional rock mass disturbance in open pit slopes related to mining excavation’, Slope Stability 2018 – XIV International Congress on Energy and Mineral Resources, Asociación Nacional de Ingenieros de Minas, pp. 1273–1288.
Seedsman, RW 2017, ‘The spalling limit in transversely isotropic materials, the prediction of the maximum height of failure, and the design of long-tendon roof support systems’, Mining Technology,
Stead, D & Eberhardt, E 1997, ‘Developments in the analysis of footwall slopes in surface coal mining’, Engineering Geology, vol. 46, pp. 41–61.

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